(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of forming air gaps under metal RF inductors that are used in the integration of RF Integrated Circuits on silicon substrates.
(2) Description of the Prior Art
In the fabrication of semiconductor devices, these devices can typically perform functions of digital data manipulation and data storage. In addition, these devices can also be used to serve as Radio Frequency (RF) amplifiers. Conventional RF amplifiers use a tuned circuit that consists of inductive and capacitive components. This tuned circuit can be used as a filter to filter out or remove signals of certain frequencies or to remove noise from a circuit configuration. The tuned circuit can also be used to form a high electrical impedance by using the LC resonance of the circuit and to thereby counteract the effect of parasitic capacitances that form part of a circuit. There are a number of significant advantages in integrating on one semiconductor monolithic substrate the functions of data manipulation and data storage with the indicated functions that are typically performed by a RF amplifier. For instance, manufacturing costs can be significantly reduced while power consumption for the combined functions is also limited. Inductors however typically are of significant size and therefore require a significant area of the semiconductor device to be implemented. The inductors are typically formed on the surface of a substrate in a spiral form. The physical size of the inductor that is created in this manner however makes the appearance of a significant amount of parasitic capacitances unavoidable. These parasitic capacitances exist between the inductor wiring and the underlying substrate. These parasitic capacitances have a serious negative effect on the functionality of the created LC circuit by sharply reducing the frequency of resonance of this circuit.
It is, as previously stated, desirable to be able to create the required inductance on the same monolithic substrate on which additional semiconductor devices are created (in order to realize the advantages that have been stated above). The parasitic capacitances that occur as part of this creation however limit the quality factor of the inductor that can be achieved using the conventional silicon process to about 10. This limitation is, for many applications, not acceptable. Dependent on the frequency at which the LC is designed to resonate, significantly larger values of quality factor, of for instance 100 or more, must be available. Prior Art has in this been limited to creating values of higher quality factor as separate units and integrating these separate units with the surrounding device functions. This negates the advantages that can be obtained when using the monolithic construction of creating both the inductor and the surrounding devices on one and the same semiconductor substrate. The non-monolithic approach also has the disadvantage that additional wiring is required to interconnect the sub-components of the assembly thereby again introducing additional parasitic capacitances and resistive losses over the interconnecting wiring network. For many of the applications of the RF amplifier, such as portable battery powered applications, power consumption is at a premium and must therefore be as low as possible. By raising the power consumption, the effects of parasitic capacitances and resistive power loss can be partially compensated but there are limitations to even this approach. These problems take on even greater urgency with the rapid expansion of wireless applications such as portable telephones and the like. Wireless communications is a rapidly expanding market where the integration of RF integrated circuits is one of the most important challenges. One of the approaches is to significantly increase the frequency of operation to for instance the range of 10 to 100 GHz. For such high frequencies, the values of the quality factor obtained from silicon-based inductors are significantly degraded. For applications in this frequency range, monolithic inductors have been researched using other than silicon as the base for the creation of the inductors. Such monolithic inductors have for instance been created using sapphire or GaAs as a base. These inductors have a considerably lower parasitic capacitance than their silicon counterparts and therefore provide higher frequencies of resonance of the LC circuit. Where however more complex applications are required, the need still exists to create inductors using silicon as a substrate. For those applications, the approach of using a base material other than silicon has proven to be too cumbersome while for instance GaAs as a medium for the creation of semiconductor devices is as yet a technical challenge that needs to be addressed.
The incorporation of RF inductors without sacrificing device performance due to substrate losses has been extensively researched in recent years. Some of the techniques that have been used for this approach include:                the selective removing (by etching) of the silicon underneath the inductor (using methods of micro-machining)        using multiple layers of metal (such as aluminum) interconnects or of copper damascene interconnects        using a high resistivity silicon substrate        employing biased wells underneath a spiral conductor        inserting various types of patterned ground shields between the spiral inductor and the silicon substrate        increasing the thickness of the inter-layer dielectric.        
The above listing of researched alternatives is not meant to be complete or all inconclusive. All of the above approaches have as common objectives to:                1) enhance the quality (Q) value of the inductor, and        2) increase the frequency of the LC self-resonance thereby increasing the frequency range over which the inductor can be used.        
The quality factor Q of an inductor can be described as follows: Q=Es/El wherein Es is the energy that is stored in the reactive portion of the component while El is the energy that is lost in the reactive portion of the component. The better the quality of the component, the closer the resistive value of the component approaches zero while the Q factor of the component approaches infinity. The quality factor for components differs from the quality that is associated with filters or resonators. For components, the quality factor serves as a measure of the purity of the reactance (or the susceptance) of the component which can be degraded due to parasitics. In an actual configuration, there are always some physical resistors that will dissipate power thereby decreasing the power that can be recovered. The quality factor Q is dimensionless. A Q value of greater than 100 is considered very high for discrete inductors that are mounted on the surface of Printed Circuit Boards. For inductors that form part of an integrated circuit, the Q value is typically in the range between about 3 and 10.
As has previously been stated, the self-resonance that is caused by the parasitic capacitance between the (spiral) inductor and the underlying substrate will limit the use of the inductor at high frequencies. Furthermore, the series spreading resistance associated with the substrate will degrade the Q factor of the inductor. This than has a direct effect on the usefulness of a monolithic or integrated inductor that is implemented on silicon substrates. This effect can be overcome if the area underneath the inductor can be made to appear locally insulating by selectively removing the underlying silicon resulting in inductors that are suspended in air. Air, having the lowest dielectric constant, forms an ideal barrier. The lowest possible and therefore the ideal dielectric constant is 1.0. This is the dielectric constant of a vacuum whereas air has a dielectric constant of slightly larger than 1.0.
The invention provides oxide fins beneath the metal inductor. The oxide fins provide the stability support for the overlying metal while also allowing horizontal air columns to simultaneously exist underneath the inductor. More than two air columns can be created by the technique presented in this invention by repetitive application of the mask used. The presence of the air wells on the surface of the substrate significantly reduces parasitic capacitances and series resistance associated with the substrate.
U.S. Pat. No. 5,828,121 (Lur et al.) shows air gaps between metal lines at different levels by etching the dielectric layers between the metal line levels. This is close to the invention.
U.S. Pat. No. 5,539,241 (Abidi et al.) and U.S. Pat. No. 5,844,299 (Merrill et al.) show air gaps under inductors.
U.S. Pat. No. 5,880,026 (Xing et al.) shows a process to form air gaps between conductors.
U.S. Pat. No. 5,880,018 (Boeck et al.) shows another air gap process between lines.